Sand Algae

[waterbear]

because, as you said, they produce a thin capsule around the algae (which I asume is the CO2 bubble that algae can produce) and from looking at the pics where the capsules do look like bubbles made of scale I really do think this is something that happens under certain water balance conditions. It is reinfored by the fact that you saw it during a specific time period (and I will guess that the calcium levels in the local fill water or the alkainity or both were higher than normal at that time). Right now in my area calcium levels are much higher than normal because of our much lower than normal water table.

[waste]

While I'm not the chemist y'all are, this sounds reasonable.

Evan, always good to 'see' you , my friend